Cobalt-base alloys

ABSTRACT

Strong, wear-resistant and corrosion-resistant products are obtainable from alloys composed of 14- 30% molybdenum, 6- 12% chromium, 0.5--4% silicon and at least 50% cobalt. This alloy contains 30- 60 volume percent of a hard Laves phase and, correspondingly, 40- 70 volume percent of a solid solution matrix phase containing about 75% by weight of cobalt.

United States Patent 1 Smith COBALT-BASE ALLOYS [75] Inventor: GaylordDarrel Smith,

Mountainside, NJ.

[73] Assignee: E. I. Du Pont de Nemours &

Company, Wilmington, Del.

[22] Filed: Aug. 16, 1974 [21] Appl. No.: 498,230

Related U.S. Patent Documents Reissue of:

[64] Patent No; 3,410,732

Issued: Nov. 12, 1968 Appl. No.: 452,382

Filed: Apr. 30, 1965 [52] U.S. Cl. 148/32; 29/1825; 29/1827;

[51] Int. Cl. C22c 19/00 [58] Field of Search 75/171, 170, 134 F, 122;

[11] E Re. 28,552

1 1 Reissued Sept. 16, 1975 Primary ExaminerR. Dean [57] ABSTRACTStrong, wear-resistant and corrosion-resistant products are obtainablefrom alloys composed of 143()% m0- lybdenum, 612% chromium, 0.5--4%silicon and at least 50% cobalt. This alloy contains 30-60 volumepercent of a hard Laves phase and. correspondingly, 40--7O volumepercent ofa solid solution matrix phase containing about 75% by weightof cobalt.

11 Claims, N0 Drawings COBALT-BASE ALLOYS Matter enclosed in heavybrackets I: appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

This invention relates to a cobalt alloy and, more particularly, to anovel, strong, wear-resistant, corrosionresistant product made from thealloy and the process for making such product.

Manufacturing and processing procedures in the fields of metalextrusion, press forging, die casting, metal coining, etc. have advancedin recent years to the point where the currently-used materials for theconstruction of equipment are restricting further advancement. Ingeneral, the equipment used in these manufacturing procedures are madeof the tool steels. These steels display satisfactory strength,toughness and fabricability but are deficient in their ability tomaintain these properties at temperatures above 1000 F. Furthermore,these steels lack sufficient oxidation and corrosion resistance.Restrictions on the use of such materials is also imposed by the factthat galling will result from rubbing contact of such materials asaluminum and brass with tool steels. Hence, a material is needed thatwill exhibit minimum galling and wear when used as a sliding surfaceunder conditions of lowlubrication, high pressure, high temperature,severe abrasion, and the like.

It is a primary object of this invention to provide strong,heat-resistant, wear-resistant, shaped articles. It is a further objectto provide a novel structural material displaying improved roomtemperature properties, some of which are substantially retained atelevated temperatures as high as 1000 F. or higher. It is a stillfurther object to prescribe at least one process for obtaining thisnovel structural material. Other objects will appear hereinafter.

The objects are accomplished, in the preferred process, by meltingamounts of cobalt, chromium, molybdenum, and silicon at a temperature of23003300 F. to provide a molten composition consisting essentially of6-12% (preferably 812%) chromium, 14-30% (preferably 26-28%) molybdenum,0.1 4% (prefera- 2 of intermetallic compounds and, secondarily, by-solidsolution alloying. This strengthening is sufiicient to permit thesealloys to be used at room temperature as well amounts of various otherphases, e.g., Co Si, Co Mog,

or various chromium or molybdenum carbides, may also be present. 7Pearson, Handbook of Lattice Spacing."

Table A contains the metallographic data on selected alloys. This tablelists the estimated volume percent of phases present, the kind ofstructure, and Knoop ([00 gram load) microhardness value. The strengthand hot hardness retention of these alloys are attributed to thepresence of at least 30 volume percent of hard Laves phase with therelatively high hardness of the solid solution as shown by the Knoopmicrohardness values. The chemical composition of the two major phaseswas determined by electron beam microanalysis. The alloy was air cast inan induction furnace and poured into a graphite mold. The compositionwas madeifrom elemental constituents and the resultant composition wasverified by wet chemical analysis. The chemical composition of eachphase was determined by an electron beam microanalyzer. This apparatuscan determine the chemical composition of grains as small as ten micronsin diameter with an accuracy of plus or minus five per- 7 cent of therelative proportion of each element present. The hard phase was found toconsist of 32.7% molybdenum, 7.7% chromium, 1.9% silicon, 1.5%manganese, 54.8% cobalt, balance'other. The hard phase was identified byX-ray diffraction as a Laves phase, structure type C-l4, of the genericformula Mo Co si. The terminal solid solution of cobalt, the matrixphase, was

found to consist of 13.9% molybdenum, 7.8% chmmium, 0.8% silicon. 1.5%manganese, 74.5% cobalt, balance other.

TABLE A.-METALLOGRAPHIC DATA ON SELECTED ALLOYS OF THIS INVENTIONChemical Composition (Weight Percent) Hard Phase Matrix Phase Co Mo CrSi C Type of Microstructure Volume Knoop Volume Knoop Percent HardnessPercent Hardness 57.6 27.8 l2.0 2.0 Nodular 40 920 60 450 62.0 28.0 8.02.0 Nodular plus laminar 40 1,481 60 735 59.7 27.9 7.9 4.0 0.5 do 601,481 40 569 62.6 28.0 8.0 0.7 0.2 Nodular plus eutectic 35 920 65 56963.4 28.0 8.0 0.1 0.5 Matrix dendrites plus eutectic 30 850 70 450 0.6Mn instead of carbon.

"().[)| B and 0.2 Zr instead of carbon.

bly 0.52%) silicon, at least (preferably at least 54%) cobalt, allpercentages being by weight; forming the molten composition into ashaped article; and cooling the article.

The novel product obtained as a result of the aforementioned process isan alloy of cobalt, chromium, molybdenum and silicon in the proportionsspecified above consisting essentially of a cobalt base solid solutionwhich is strengthened, primarily, by the formation As disclosedpreviously, the articles of the invention may be prepared by melting andcasting the novel alloy compositions using conventional castingfurnaces, molds, and techniques. Alternatively, such compositions may beprereacted and the reaction product reduced to powder size prior toconversion to shaped articles by cold pressing followed by sintering orby hot pressing at elevated pressures, or by melting and castmg.

The articles of the invention are found to exhibit a lumber ofoutstanding and surprising properties. They ire strong and hard at roomtemperature and maintain hese properties to temperatures as high as [500F. or iigher. While they exhibit no major elongation upon 'racture atroom temperature, these articles exhibit satsfactory impact strengthsunnotched and at least one hot pound of impact resistance in thestandard Charpy l-notch test. Furthermore, these alloy articles are reistant to thermal stress, degradation by heat, air, cerain corrosivemedia, and molten aluminum. In the 'o rm of dies, they are resistant togalling by the extruion of aluminum and brass. Graphite rubbing againstlrticles made from alloys within the scope of the invenion at highspeeds and pressures causes virtually no vear (i.e., less than one mi]of wear for 100 hours of )peration at 100 psi. and 3500surfaceft./min.).

These properties make the alloys suitable for a vari- :ty of criticalstructural components, e.g., dies for exrusion, pressing, coining,casting and forging gasoline md diesel engine exhaust valves, furnacefixtures,

:hemical seals, etc. The properties of these alloys also nake themuseful where a good sliding surface is deired, e.g., seal components,sleeve bearings bushings, 'alve guides, extruder liners, piston ringsand sleeves.

ln the' preparationof these products, it is preferred to isecommercially pure elements. Since only minor :hahges in the relativeproportions of the essential elenents will occur during the processing,it is possible to ta'rt with substantially the amounts of the componentshat are desired in the final product. Thus, the amounts If the elementconstituents desired in the ultimate )roduct are melted in a furnacedesigned for melting llloys in a temperature range of 23()()--33()() F.and the esulting molten composition is cast in molds or cruci- JlCS ofgraphite, cast iron, copper or' ceramics.The :omposition may be meltcast in air, vacuum or in an nert atmosphere. Conventional shell andinvestment nolds may be used for casting the shaped objects.

In a specific process,'the elemental composition is nelted in anopen-air induction furnace lined with nagnesium oxide or silicon dioxideand is cast into cast ron molds. Initially, the desired amounts ofcobalt and .hroniium are melted, after which the molybdenum ind half ofany silicon may be added. The amount of .ilicon added in this step isonly the amount necessary 'or use as a deoxidizer. lf vacuum melting isused, the .ilicon need not be added at this point. The final porionofsilicon, which is the amount desired in the ultinate product, is addedjust prior to pouring the alloy :omposition. Any other alloy additionscan also be nade at this time. Thus, calcium-silicon, ferro-silicon, )rferro-manganese may he added to deoxidize the illoy or the conventionalhot-topping compound may e used to minimize porosity and pipe in thearticles :ast. It should be understood that the molybdenum and :hromiummay be added in the form of the cheaper fero-molyhdenum andferro-chromium alloys. However, he total iron, which can substitute forpart of the co- !altin excess of 5071 by weight of cobalt should not ex-:eed 1071. Morethan l 7c iron results in altering the nigh temperaturestrength and hardness of the alloys.

As disclosed previously the percentages ofcompotents in the finalcomposition should be essentially as ollows: 6-l27r chromium, 14-30%molybdenum, 11- 1% silicon, the remainder being cobalt in an mount of atleast 50%. Alloys outside these ranges are significantly deficient inone or more of the previously mentioned properties of the alloys of theinvention. At

. least 0.1% silicon is necessary for minimum deoxidation and fluidityof the alloy during casting. Up to 4% silicon tends to enhance specificalloy properties, presumably by promoting the formation of Laves phase,but more than 4% silicon results in alloys that form brittle articles.Additions of more than 1.0% carbon also tend to cause excessivebrittleness. Hence, it is preferred'that the'alloy contain 0.02O.5%carbon as a contribution to wear resistance. Up to 2% manganese enhancestoughness of the alloy by reducing the tendency for sulfurembrittlement; Other elements may be added to the compositions of thisinvention provided they do not have a substantially adverse effect uponone or more of the desirable properties. For example, it has been foundthat the iron, boron, and zirconium may be added in limited amounts forcertain purposes. The addition of iron has been discussed. Boron andzirconium additions have been found to enhance the high temperaturestrengths of the alloys.

Another method of alloying the components and forming the shapedarticles is to first premelt the composition, thenreduce the resultingalloy to a powder and, thereafter, convert the powder to a shapedarticle. Premelting of the essential elements tends to insure uniformityof composition in the final alloy article. The premelting step involvesarc-melting or inductionmelting of the untreated composition followedeither by atomizing to form the particulate material directly or bycasting into an ingot and reducing the ingot to a powder. Reducing to apowdermay be accomplished by jaw crushing the material to minus 4 meshfollowed by milling in a steel or tungsten-carbide lined equipment to afine particulate size, Le, of which will pass a' minus 240 mesh screen.

The powders may readily be shaped by cold pressing in steel dies atpressures of from about 10 to 50 tons/sq. in. The cold pressed objectcan then be sintered or liquid phase bonded at temperatures between 2100F. and 2500 F. for a period from less than one minute to as much asminutes. The liquid phase bonding temperature is between the solidus andliquidus line for these alloy compositions. lnert gas, hydrogen, orvacuum furnace atmospheres are satisfactory and will yield dense,bright, shaped objects.

Shaped articles can also be made from the powders by hot pressing thepowder in graphite dies at temperatures between 2000 F. and 2400 F at1000 lbs/sq. in. or higher. Atmosphere control in this operation is notcritical. Soaking time at the operating temperature is dependent uponthe mass of the object but will usually be limited to times in the rangeof about 5 to 20 minutes.

The following illustrative examples constitute specific embodiments ofthis invention and are not intended to, be limitative. In theseexamples, various property data are reported. Thetest methods by whichthese data are obtained are. unless otherwise stated, the standard ASTMtest methods using standard ASTM specimens.

Examples [-9 The following representative series of alloys within thescope of this invention are prepared:

TABLE 11 Example Room 1.000 F. 1,200 F. 1 500 F. 1,1450 F.

Temp.

2 84 73 79 66 2s 3 91 8x 90 86 33 7 122 125 1 13 s 28 9 135 130 98 9| 25Example C0 M0 CT In Table III-A, the hot hardness data for some of the 1576 278 120 20 06 alloys are compared to data on two commercial tool 259.7 27.9 7.0 4.0 0.5 steels: Tool Steel A composed of 73.9% Fe. l271Cr, 3; 2;? 5;; 3-3 f}, 8-2 12% w, 1% v, 0.5% Si, 0.3% c and 0.3%Mn andT001 5" 63.6 25.6 7.9 1.8 0.6 Steel B composed of9l% Fe, 5.2% Cr, 1.3%Mn, 1.1%

II g gig 53-2 Z3 ff 8*; v, 1% s1 and 0.4% c. The ab1l1ty of the alloysof the 1n- 8 64.1 25.6 7.9 1.8 0e vention to retain their high hardnessvalues as tempera- 9 tures are elevated compared to the inability of thetool "Also contained 0.l7'/: B and 0.9% Zr. Steels should be noted'"Also contained L09: C.

"Also contained 0.5% C. "Also contained 0.37: C. In preparing thesealloys, 1 0 lbs of the elemental Exumpk Ra 70 Ru 6U Ra 50 Ru 40composltlon is melted in an induction furnace lined with MgO and castinto graphite molds 4" wide X 8 2 1,120 F 1,2s0 F 1 390 F 1518 F. deep.There is no reaction between the alloys and the g A F 2:1 graphite.Initially, the cobalt and chromium are melted steel 3 1.010" F 1,150 F1:190 F 1300" F: and the molybdenum and half the silicon are then added.The final portion of silicon is added just prior to pouring the alloy.Any other alloy additions are also made at this time. The alloy ispoured from the furnace into a ladle. Calcium-silicon is added to theladle to flux Furthermore the hardness Values of the alloys of the thealloy and deoxidize the composition. The pouring inventlon aresubstantially unaffected by the length of temperature of 2800-3000 F. ismeasured by an optitime that the alloys are p at the elevated p calpyrometer immediately before pouring the molten Thls Show Table metalinto the graphite mold. Vermiculite is used to cover the castinimmediate] after ourin to ermit g y p g p TABLE Ill-B slower cooling ofthe ingot. Test specimens are machined from the ingots usingconventional tooling pro- 40 a cedures. Conventional grinding wheels andcarbide E l H Ru 7 tools are sufficient for all machining operations. Lmmum hour Table I lists the Rockwell A hardness (R,,), the ultix 53 e354 mate tensile strength at room temperature (UTS), the Steel A 40unnotched impact strength (IS), the Charpy V-notch impact strength(CIS), and the results of cycling a 4" X 4 X 1" specimen in two examplesfrom 400 F. to 18500 and back to Within 30 seconds, 100 Table IV liststhe time-to-rupture data for the alloys times of Examples 3, 7 and 9.The advantages of the boron and zirconium in the alloy of Example 3 areapparent.

TABLE 1 Example R U.T.S. LS. C.l.S. Effect of A (X10 p.s.i.) (ft. lbs.)(ft. lbs.) Cycling TABLE V A g2 l N 55 Temp. Stress Time Elongation ODEs v 3 74 75 9] 3.0 1.0 Example F.) (p.s.1.) (hrs) (pnerlc cnt 4 79-803.0 1.0 2 32:33 3: 1'8 3 1,200 80.000 315.4 5.4 7 74 76 122 1b None1,350 35.000 909.8 20.2 8 7646 60 7 1,200 80.000 49.8 4.2 9 78 79 1.20070.000 214.0 5.6 1,350 30,000 288.7 5.7 1,500 25,000 19.5 5.5 9 1,20080,000 3.6 2.9 Table II shows the effect of temperature on ultimate1.200 60000 2700 tensile strength for some of the alloys. 1,350 30,00059.4 8.2

TABLE lV-Continued l'emp. Stress Time Elongation Example F.) (p.s.i.)(hrs.) (percent in l The corrosion resistance of the alloys of 'theinvention I compared to a Control prepared from an alloy composition of53.6 C0, 27.8 Mo, 16 Cr, 2 Si and 0.6 Mn are presented in Table V-A. Thecriticality of the 12% limit on chromium, particularly ,withhydrochloric acid, will be apparent] TABLE V-A Av. Cor. Penetration Rate(mils/mo.) at 110 F.

Example 107: H 50. 7892 H 50 1071 HCl 37?! HCl' 1 0.25 0.89 16.0 4.39 20.70 20.90 19.5 6.05 8 0.49 0.00 8.25 0.42 9 0.60 0.29 2.76 0.20[Control 0.22 0.90 Not resistant] The oxidation resistance of the alloysof the invention are compared to commercial materials currently used asoxidation-resistant materials in Table V-B. Thus, Control A is gray castiron (94 Fe, 3.25 C, 1.75 Si, 05 Mn. 0.35 P and 0.1 S) which is now usedby the die casting industry to contain aluminum; and Control B is acommercial oxidation resistant alloy of 74.5 Ni, 15 Cr, 7 Fe, 2.5 Ti and1 Ch. i

TABLE V-B Dissolution by Resistance to Still Air To illustrate theapplicability of powder metallurgy techniques as a means of preparingalloys hereof, a l-pound charge of 6271 Co, 28% Mo, 87r Cr, 2% Si isatomized to minus 100 mesh powder. The atomization is accomplished byinduction melting the charge and spinning the stream of molten materialfrom a copper plate. The particles solidify as spheres as they arequenched by a stream of nitrogen or inert gas, such as argon. The chargeyields 100% of minus 100 mesh powder of which 40% is minus 230 mesh. Tofurther reduce the powder, a -pound charge is ballmilled dry for tenhours at 60 r.p.m. to obtain at least 95% minus 325 mesh powder. Themill is a four-quart steel mill (8 inches diameter) and containsapproximately five pounds of tungsten carbide inserts X /2" X V2").Solid bars X /2" X 2") are then made by hot pressing the composition ingraphite dies at pressures ranging from 0.5 to 1.5 tons/sq. in. andtemperatures between 2100 F. and 2250 F. for periods of one minute toapproximately twenty minutes. The maximum R hardness is 82-84 and theaverage room temperature ultimate tensile strength for these bars is130,000 p.s.i.

Examples 1 1-13 The advantages of using the alloys of the invention inlieu of tool steels in high temperature applications is illustrated bycomparing the performance and operating characteristics of each typematerial in actual service. Results of aluminum extrusion, brassextrusion, and steel coining of engine valves using both type materialsis shown below.

The aluminum extrusion experiments of Example 1 l are conducted on a2,500 ton hydraulic tube extrusion press. This press requires a castaluminum billet 9" dia. X 28" long. The initial work is the extrusion of1.6" dia. tube (1.375 dia. internal dia.) using 6063 aluminum (0.4% Si.0.7% Mg, Bal Al). The billet preheat temperature is 800 F. The extrusiontooling is preheated to 400 F. Using nitrided commercial tool steel(91.05% Fe, 5.25% Cr, 1.25% Mo, 1.00% Si, 1.05% V, 0.40% C) for theextrusion die and mandrels results in an average maximum extrusion speedof 225 ft./min. Although a graphite-oil lubricant is applied to the dieface prior to each extrusion, the dies usually pick up sufficientaluminum on the bearing of the die to start scratchingand galling of theextrudate, thus resulting in poor surface finish. These commercial toolsteel dies require cleaning on the average of every sixth extrusion. Thedie is cleaned by using a small emery drill at tached to the shaft ofahand-held drill tool. This action eventuallyresults in the rounding ofthe edges of the diethereby' necessitating either scrapping or reworkingof the die; Commercial cobalt base alloys, have been unsatisfactory forthis application due to rapid failure by cracking under load or heatshock.

A die insert of 64.81% Co, 25.6% Mo, 7.19% Cr, 1.89? Si,0.67( Mn isfabricated from an air cast 3" dia. slug for insertion in a commercialtool steel die case. The insertand case are employed in the same mannerasthe tool steel die. A total of 187 billets are extruded through theinsert at 330 feet per minute without any cleaning being requiredbecause of aluminum pick-up or galling on the bearing face.

Free machiningbrass is used to demonstrate the extrusion toolcapabilities of the alloys of this invention for extruding copper basealloys in Example 12. Two inserts for a two part die holder are machinedfrom air cast 3" dia. slugs of the composition 64.1% C0, 25.6% M0, 7.9%Cr, 1.8% Si, 0.6% Mn. The inserts are designed to extrude a 0344' roundbrass rod. The entrance design of the inserts has a straight 30 includedanglecone leading directly into the bearing area. The

temperature of the billets is 1450 F. The press liner is 800 F. and thedies are heated to 900 F. Twenty 8 dia. by 32" long billets are pushedthrough the twoinsert die tool and tool wear is measured for comparisonwith commercial tool steel wear. No tool wear or flow of metal isobserved for the inserts made from the cobalt-base alloy. Up to 0.060"closure can normally be experienced for a tool steel (73.97: Fe, [2.00%Cr, 0.30% Mn, 0.50% Si, 12.00% W, 1.00% V, 0.30% C) conventionallyemployed for such service under the same conditions in five extrusions.

To evaluate the alloys hereof as steel extrusion tool material inExample 13, an automatic valve toggle press is selected. This equipmentautomatically ex trudes steel valve stems and then coins the valve tofinal shape. The dies conventionally employed are made of a tool steel(91.05% Fe, 5.25% Cr, 1.25% Mo, 1.00% Si, 1.05% V, 0.40% C) and lastbetween one or two hours during which time between 1,000 and 2,000valves are produced depending on whether the valve being made is anintake valve (SAE 1047 steel) or an exhaust valve (20 to 22% Cr, 1.475to 0.575% C, 8 to Mn, 3.5 to 4.5% Ni, Bal. Fe). Each machine has twodouble die sets; a set consisting of one extrusion die and one coin die.

The heated slug of steel (1850 F. for plain carbon steel and 2150 F. foralloy steel) is dropped into the extrusion die where the stern of thevalve is extruded and part of the unextruded material is left at the topof the stern. In the next operation, the extruded valve is lifted offand placed into the next cavity which contains the coining die. The stemof the valve easily passes through the hole in the coining die and asthe punch comes down, the mass of metal on top of. the stem is flattenedout to form the head of the valve. In the meantime, the next heated slughas dropped into the extrusion die cavity for extrusion. Toggle pressesoperate at the rate of 16 strokes/min, in forming the exhaust valves;and about 32 strokes/min. in forming the intake valves. Voluminousamounts of water-base graphite lubricant is used in the dies. Failureoccurs when the dies are washed out. 7 I

An exhaust coining die machined from an air cast billet of 64. 1% Co,25.6% M0, 7.9% Cr, 1.8% Si, 06% Mn (the alloy of the invention) produces2,800 valves. The die fails by cracking. The valves coined by the diemade of this alloy are dimensionally exact. In contrast to this, theconventional tool steel dies have a life of only 1 ,000 to 2,000 valvesand show gradual wear during their life causing the valve-headdimensions to vary.

Example 14 The corrosion resistance and the high hardness values ofarticles of the alloys of this invention suggest the use of this alloyin a number of industrial applications that involve both wear andcorrosion. One such application is a chemical pump seal and anotherapplication is a grinder wheel in an impact mill for breaking upchemical sludge. A simulated performance test for evaluating candidatematerials as mechanical seals comprises rotating a 2 /2" dia. sealagainst a graphite stationary seal at a speed of 3,500 r.p.m. and apressure of 100 p.s.i. for 100 hours using water as the liquid mediurn.The controlled variables are the pressure exerted on the seal faces andthe water temperature. The measurement of the total wear (carbon sealwear plus mechanical seal wear) is used as the performance criteria. Awear rate of 0.00083" or less per 100 hours is acceptable; anything overthis is considered a failure. This wear rate is based on actual lifeperformance of the seal assembly of Va" in 15,000 hour service. Underthese conditions, 85 p.s.i.g. pressure has been established as themaximum for almost 95% of the seal materials now in use. Among thesematerials are stainless steel and aluminum oxide.

A test seal is machined from an air cast billet X 4" X 6") of thecomposition 63.8% C0, 25.6% M0, 7.9% Cr, 1.8% Si, 0.6% Mn, 0.3% C. Thebearing surface is lapped to within 10 lightbands of flatness and thenpolished on metallographic felt laps to bring the surface of the sealinto relief. The relief is a result of the relative hardnesses and wearrates of the several phases. The relief polish causes the harder phasesto stand out above the softer matrix phase. The relative height of theharder phases above the matrix depends on the extent of polishing and onthe composition selected for relief polishing. Approximately fifteenminutes of relief polishing results in a relief of approximately 5 to10' microns. A mixture of 10 ml. HNO 20 ml. HCl, 40 ml. glycerol appliedfor30 seconds to the polished surface enhances the relief effect. Thisrelief polishing has been found necessary for the cobalt-base alloyshereof to permit immediate smooth operation of the seal and to minimizecarbon wear during the breaking-in period. It

p.s.i.g. pressure a stainless steel seal exhibits a wear ofapproximately 0.001 inch.

Example 15 Alumina coated nickel-base alloy (62% Ni, 30% Mo, Bal. Fe) isfound to have a wear rate of 23 mils per year in a simulated, test ofthe action of a grinding wheel on an impact mill breaking up the bottomsludge in a cooling vesselhThe simulated test duplicates the environmentwhich contains benzonitrile, copper cyanide. amine hydrochlorides, andsome fluorides and consists of rotating a test specimen in the sludge at3450.r.p.m. for .210 minutes at 212 F. A test coupon machined from anair cast billet (*A" X 4 X 6") of 64.1% C0. 25.6% M0, 7.9% Cr, 1.87: Si,0.6% Mn, representing an alloy of the invention, exhibits an erosionrate of 7 mils per year.

Example 16 To demonstrate the criticality of the compositional limits ofthe alloys of this invention, the components of an actual conventionalseal operating with a carbon graphite sleeve on a two inch diametershaft are made from alloys within and outside the compositional limits.The shaft speed is 2500 r.p.m. The seal fluid is water at p.s.i.g. andthe ambient temperature is 100 to F. The seal pressure can be variedfrom 50 p.s.i.g. to 300 p.s.i.g The test time is 100 hours and the totalwear of the carbon-graphite mating material plus the metal surface isused as the performance criteria. The seal face temperature is measuredas a means of monitoring the seal perfomiance in operation. Erratictemperature fluctuations and high seal face temperatures are anindication of rapid seal face wear.

A stellite weld o erlay on steel, conventionally used in a seal, causesthe carbon-traphite to wear at the maximum allowable rate of 0.00083inch per 100 hours at a pressure of 50 p.s.i.g. under the conditionsdescribed above. A sea] component, machined from an air cast billet of26% molybdenum, 9.0% chromium, 1.8% silicon, 0.6% manganese, no carbon,and the balance cobalt, causes the carbon-graphite to wear only 0.000]inch in 100 hours. Thus, the above test clearly indicates thesuperiority of the alloys of this invention over the currently employedstellite overlays as a mating face against carbon-graphite in waterunder the above conditions. The alloy component exhibits no wear.

Using the ,same alloy composition, but increasing the seal pressure to100 p.s.i.g. increases the wear of the carbon-graphite seal component ina duplicate test to .044 to .048 inch to l00.hours. But this greaterwear rate can be overcome by adding carbon to the alloy. That carbon isbeneficial at higher seal operating pressures is shown by the fact thatthe above alloy containing an additional 0.36%. carbon causes thecarbongraphite to wear only 0.0001 inch in l hours at l00 p.s.i.g.

That the chromium content is critical at higher seal operating pressuresis shown by the fact that an alloy containing 28% molybdenum, 2.0%silicon, balance cobalt and chromium wass tested at three chromiumcontents: none, 8% and 16%. At l00 p.s.i.g. and 100 hours under theabove conditions the carbon-graphite wear is 0.044, 0.0001, 0.136 inchrespectively. The limits on the molybdenum content are similarlycritical.

What is claimed is:

1. An alloy composition consisting essentially of at least 50% cobalt,l4-30% molybdenum, 6-l2% chr0- mium and 0.5-4% silicon, said alloyhaving a microstructure containing 30-60 volume percent of a hard Lavesphase and, correspondingly, 40-70 volume percent of a solid solutionmatrix phase.

2. An alloy composition as in claim 1 amount of cobalt is at least 54%.

3. An alloy composition, as in claim amount of molybdenum is 26-28%.

4. An alloy composition as in claim 1 amount of chromium is 8-1271.

5. An alloy composition as in claim 1 amount of silicon is 0.5-271.

6. An alloy composition consisting essentially of at least 50% cobalt,l43()7r molybdenum, 642% chromium, (LS-4% silicon, up to 10%1ron, up to2% manganese and up to l% carbon, said alloy having a microstructurecontaining 30-60 volume percent of a hard Laves phase and,correspondingly, 40-70 volume perwherein the 1 wherein the wherein thewherein the cent of a solid solution matrix phase.

7. An alloy composition as in claim 6 wherein the amount of carbon is02-05%.

8. A powder metallurgy composition consisting essentially of fineparticles of an alloy having a size wherein 95% pass a minus 240 meshscreen, said alloy consisting essentially of at least 50% cobalt, 14-30%molybdenum, 6-l2% chromium and 0.5-4% silicon, said alloy having amicro-structure containing 30-60 volume percent of a hard Laves phaseand, correspondingly, 40-70 volume percent of a solid solution matrixphase. 9. A powder metallurgy composition considering essentially offine particles of an alloy having a size wherein 95% pass a minus 240mesh screen, said alloy consisting essentially of at least 54% cobalt,26-28% molybdenum, 8l2% chromium, O.5%2% silicon and 02-05% carbon,saidalloy having a micro-structure containing 30-60 volume percent of ahard Laves phase and, correspondingly, 40-70 volume percent of a solidsolution matrix phase.

10. A shaped article consisting essentially of two phases from'an alloycomposition, 30-60 volume percent of a Laves phase having the genericformula Mo Co Si and, correspondingly, 40-70 volume percent of a secondphase of a solid solution of a matrix phase of a composition containingabout by weight cobalt, said alloy consisting of at least 50% cobalt,l4-30% molybdenum, 6-l2% chromium and 0.5-4% silicon.

l]. A shaped article consisting essentially of two pha ses from an alloycomposition, 30-60 volume percent of a Laves phase having the genericformula Mo Co Si and, correspondingly, 40-70 volume percent of a secondphase of a solid solution of a matrix phase containing about 75% byweight cobalt, said alloy consisting essentially of at least 54% cobalt,26-28% molybdenum, 8-1271 chromium and (LS-2% silicon.

1. AN ALLOY COMPOSITION CONSISTING ESSENTIALLY OF AT LEAST 50% COBALT,14-30% MOLYBDENUM, 6-12% CHROMIUM AND 0.5-4% SILICON, SAID ALLOY HAVINGA MICRO-STRUCTURE CONTAINING 30-60 VOLUME PERCENT OF A HARD LAVES PHASEAND, CORRESPONDINGLY, 40-70 VOLUME PERCENT OF A SOLID SOLTION MATRIXPHASE,
 2. An alloy composition as in claim 1 wherein the amount ofcobalt is at least 54%.
 3. An alloy composition as in claim 1 whereinthe amount of molybdenum is 26- 28%.
 4. An alloy composition as in claim1 wherein the amount of chromium is 8- 12%.
 5. An alloy composition asin claim 1 wherein the amount of silicon is 0.5- 2%.
 6. An alloycomposition coNsisting essentially of at least 50% cobalt, 14- 30%molybdenum, 6- 12% chromium, 0.5- 4% silicon, up to 10% iron, up to 2%manganese and up to 1% carbon, said alloy having a micro-structurecontaining 30- 60 volume percent of a hard Laves phase and,correspondingly, 40- 70 volume percent of a solid solution matrix phase.7. An alloy composition as in claim 6 wherein the amount of carbon is0.2- 0.5%.
 8. A powder metallurgy composition consisting essentially offine particles of an alloy having a size wherein 95% pass a minus 240mesh screen, said alloy consisting essentially of at least 50% cobalt,14- 30% molybdenum, 6- 12% chromium and 0.5- 4% silicon, said alloyhaving a micro-structure containing 30- 60 volume percent of a hardLaves phase and, correspondingly, 40- 70 volume percent of a solidsolution matrix phase.
 9. A powder metallurgy composition consideringessentially of fine particles of an alloy having a size wherein 95% passa minus 240 mesh screen, said alloy consisting essentially of at least54% cobalt, 26- 28% molybdenum, 8- 12% chromium, 0.5%-2% silicon and0.2- 0.5% carbon, said alloy having a micro-structure containing 30- 60volume percent of a hard Laves phase and, correspondingly, 40- 70 volumepercent of a solid solution matrix phase.
 10. A shaped articleconsisting essentially of two phases from an alloy composition, 30- 60volume percent of a Laves phase having the generic formula Mo2Co3Si and,correspondingly, 40- 70 volume percent of a second phase of a solidsolution of a matrix phase of a composition containing about 75% byweight cobalt, said alloy consisting of at least 50% cobalt, 14- 30%molybdenum, 6- 12% chromium and 0.5- 4% silicon.
 11. A shaped articleconsisting essentially of two phases from an alloy composition, 30- 60volume percent of a Laves phase having the generic formula Mo2Co3Si and,correspondingly, 40- 70 volume percent of a second phase of a solidsolution of a matrix phase containing about 75% by weight cobalt, saidalloy consisting essentially of at least 54% cobalt, 26- 28% molybdenum,8- 12% chromium and 0.5- 2% silicon.